Various silicone emulsion compositions have hitherto been proposed which form elastomers after the removal of water. For example, JP-B-38-860 proposes a silicone emulsion composition comprising a polydiorganosiloxane whose molecular chain has been blocked at each end with a hydroxyl group, a polyorganohydrogensiloxane, a poly(alkyl silicate), and a tin salt of a fatty acid. (The term "JP-B" as used herein means an "examined Japanese patent publication".) JP-B-57-57063 proposes a silicone emulsion composition comprising a polydiorganosiloxane whose molecular chain has been blocked at each end with a hydroxyl group, a silane having a functionality of 3 or higher, and a tin salt of a fatty acid. JP-B-58-17226 proposes a silicone emulsion composition comprising a polydiorganosiloxane whose molecular chain has been blocked at each end with a vinyl group, a polyorganohydrogensiloxane, and a platinum compound. Furthermore, JP-A-54-131661 proposes a silicone emulsion composition obtained by emulsion-polymerizing a cyclic organosiloxane with an organoalkoxysilane having a functional group. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
However, the elastomers formed from those emulsion compositions have poor mechanical strength although they are excellent in heat resistance, water repellency, weatherability, transparency, etc. Those prior art emulsion compositions have therefore been unsuitable for use as, e.g., a coating material.
For improving the poor mechanical strength, several proposals have been made on techniques of adding a colloidal silica as a reinforcement. For example, JP-B-62-54824 discloses the addition of a tin salt of a fatty acid and an alkaline colloidal silica to an emulsion of a polydiorganosiloxane whose molecular chain terminals have been blocked with hydroxyl groups. In JP-A-61-16927 is disclosed the addition of an alkaline colloidal silica to a silicone emulsion obtained by the emulsion polymerization of a low degree of polymerization diorganosiloxane and blocked at each end with a hydroxyl group with an alkoxysilane having a functionality of 3 or higher.
However, the elastomers formed from the above prior art compositions by removing the water therefrom have insufficient interfacial adhesion between the colloidal silica and the polyorganosiloxane and have poor uniformity in dispersion of the colloidal silica. Thus, the reinforcing effect of the silica on the silicone is insufficient. The above emulsion compositions further have a drawback that the pH thereof should be kept in the range of from 9 to 12, in which the alkaline colloidal silica added is stable, in order to stably store the emulsion compositions over a prolonged period of time. This pH regulation is troublesome.
In order to mitigate the problem described above, JP-A-61-16929 and JP-A-61-271352 disclose a process for producing an emulsion which comprises emulsion-polymerizing a polyorganosiloxane having a low degree of polymerization and terminated at each end with a hydroxyl group with an alkoxysilane having a functionality of 3 or higher in the presence of an acid colloidal silica.
The above prior art process, however, has a drawback that it is difficult to conduct initial homogenization so as to convert a mixture of the starting-material siloxane and the colloidal silica into micelles each comprising the two ingredients. As a result, the reaction mixture gives an emulsion containing particles formed by the condensation of the siloxane with silica within micelles and, coexistent with these particles, siloxane and silica particles which have not participated in the condensation. Thus, a sufficient improvement in mechanical strength is not attained.
A further drawback of the emulsion compositions described in the references cited above is that they are not expected to cure so rapidly upon evaporation of the volatile components by heating.
On the other hand, attempts have been made in recent years to use a particulate silicone as a modifying additive for relieving the internal stress of various organic resins and for improving the impact strength thereof.
A conventionally employed process for producing a particulate silicone for the above use is to pulverize a silicone rubber either as it is or after freezing the same. However, this conventional process has problems, for example, that the production efficiency is low and that the rubber particles obtained have a wide range of large particle diameters, are irregular in shape, and have poor dispersibility and poor lubricity.
For mitigating the above problems, various methods have been proposed.
For example, JP-A-59-68333 and JP-A-63-17959 disclose a method comprising spraying a liquid silicone rubber composition in hot air to cure the rubber in the sprayed state to thereby obtain spherical cured particles. In JP-A-62-243621, JP-A-62-257939, JP-A-63-77942, and JP-A-63-202658 is disclosed a method which comprises converting a liquid silicone rubber composition into an aqueous emulsion or suspension and bringing the emulsion or suspension into contact with a high-temperature liquid or gas to cure the silicone to thereby obtain spherical particles.
However, the particulate materials obtained by the above methods have poor mechanical strength because they do not contain a reinforcing filler such as various silicas and fumed titanium oxide. These particulate materials hence have a drawback that when used as an additive for modifying organic resins, the particulate materials change in shape upon application of an external stress thereto and may finally break. If a reinforcing filler is incorporated into the liquid silicone rubber composition in order to mitigate the above drawback, this not only impairs flowability but also results in a particulate material having an increased particle diameter and irregular shapes due to the aggregative properties of the filler.